Mineral identification is essential to most geoscience investigations including research (field and experimental), industrial, and regulatory investigations in the fields of mineralogy, petrology, geochemistry, geochronology, petroleum exploration, mineral exploration, mining geology, gemology, mineral processing, building materials, environmental health, medical mineralogy, forensic mineralogy, planetary geology, and more. Meeting the needs of all of these industries and applications requires analytical technology that is dependable, accurate, capable of handling a wide range of physical requirements (sample size, sample format, sample location), accessible to users having a wide variety of training, and, ideally, affordable.
Routine mineral identification (“RMI”) can be defined as an analytical process that provides for identification of a mineral sample with a low level of sample and/or instrument preparation for each analysis, with relatively quick results, and with a high level of dependability (repeatability and probability of accurate results). However, when an analytical process is highly expensive, requires high amounts of physical space and/or physical support, requires high levels of maintenance and calibration, and/or requires a high level of education and training to operate, it is also impractical—it cannot be widely deployed to or readily available to a high percentage of those individuals/institutions who routinely need analytical results. As such practical RMI (“PRMI”) can be defined as an RMI system that requires moderate to low amounts of user training, cost, space, and maintenance.
While individuals with sufficient training and experience may be able to identify many minerals with a few hand tools, there is a widespread need for more analytical and automated means of routine mineral identification. The next level of technology traditionally employed is polarized light microscopy, however, this requires considerable sample preparation and continues to require considerable training and experience. Since the middle of the 20th century the primary technological tool employed for RMI has been powder X-ray diffraction (“PXRD”). Modern improvements have reduced the physical size and cost of PXRD instrumentation and computers have provided for considerable automation of the analytical process, but its application generally requires extraction and powdering of the mineral sample. This type and amount of sample preparation means that PXRD requires a sample-preparation delay before each analysis and is extremely difficult-to-impossible to apply to small mineral grains or mineral grains in situ (without extraction). Additionally, despite the reduction in instrument size that has been achieved for PXRD, it continues to be impractical to attempt to employ PXRD out into a field investigation environment.
In contrast to PXRD, Raman spectroscopy can be employed with little to no sample preparation, in situ, and on physical scales down to the micron level. Raman instruments also typically require relatively little space and both field portable and hand-held units are available. However, a general lack of optimization of the Raman technology for application to minerals (and man-made crystalline solids as well) has prevented widespread adoption of Raman technology for RMI.
The Raman Effect produces a change (shift) in the wavelength of light scattered by a substance. This shift is the result of light interacting with the quantized energy levels of molecular vibrations—as such the magnitude of this wavelength shift and the intensity of Raman scattering are directly related to the mass of the atoms, the nature of bonding forces between neighboring atoms, and the geometric (symmetry) relationships between neighboring bonds. In other words, the spectral position and intensity of the peaks in Raman spectroscopy result from the structure and chemistry of the subject substance. Unlike many other spectrometries, however, the response from the atoms/molecules is not at specific characteristic wavelength but is rather at a specific shift (spectral distance) from the wavelength of the excitation source—generally a laser. As a result Raman instruments using excitation lasers at different wavelengths will produce the same pattern of Raman shifts from the same substance.
Some additional aspects of the interaction of light with matter become important when attempting to achieve dependable results from Raman systems: (1) The Raman Effect is actually a very low probability interaction—much of the incident light is elastically scattered (without a change in wavelength). (2) Although the pattern of Raman shifts from a particular target substance is independent of the wavelength of the excitation laser, the intensity of the Raman peaks are not. The probability and, therefore, the intensity of Raman scattering increases steadily as the wavelength of the excitation source decreases. (3) Some of the energy of the incident light is absorbed and converted to thermal energy. Depending on the amount absorbed by any specific target substance and its thermal conductivity and thermal stability, the target substance may be modified or even destroyed by the Raman laser. (4) Last, but far from least, for many substances some of the absorbed light energy is re-emitted as longer-wavelength light—an effect referred to as fluorescence or photoluminescence. When a target substance responds to the Raman laser with fluorescent light its intensity tends to be stronger than Raman scattered light, often orders of magnitude stronger. Unlike the Raman Effect, however, the intensity of fluorescence, even whether it occurs at all, does depend upon the wavelength of the excitation light source.
The basic components of a Raman spectrometer system are as follows: (1) a laser as the excitation light source since the need to measure wavelength shifts and the low probability of the Raman Effect leads to a need for a high intensity monochromatic excitation source; (2) optical components to focus the laser beam onto the sample; (3) optical components to direct the light scattered/emitted from the sample towards a spectrograph; (4) a long-pass or notch filter positioned between the sample and the spectrograph to separate the wavelength-shifted light from elastically scatted light at the incident wavelength; and (5) a spectrograph typically employing a diffraction grating and a solid-state array detector.
For standardized sample holders without spatial specificity like liquids in a cuvette, mechanical sample positioning is sufficient. When spatially specific sample positioning is required some kind of optical pointing system is required—often this is a microscope with the laser source coaxially introduced with partially reflecting mirrors. Finally, for reasons ranging from physical convenience to a practical need to separate the spectrometer from the sample, many Raman spectrometers employ a fiber-optic bundle with the laser source being delivered through a central fiber and scattered light being carried back to the spectrometer through the remaining fibers in the bundle. In general, again due to the weakness of the Raman Effect, the sample must be isolated from any ambient light.
The performance challenges of Raman technology often intersect or even compete with one another resulting in performance trade-offs. The central performance challenges are the minimum Raman shift, spectral resolution, spectral range, signal sensitivity, and reducing/avoiding fluorescence interference.
The minimum Raman shift is determined by the type and quality of the cutoff filter. This performance parameter is only critical when the Raman spectrum of the materials being studied has important or critical peaks in the low Raman shift region (e.g., less than 300 cm−1).
The technology required to increase spectral range generally reduces resolution. This trade-off can be reduced by increasing the physical size of the spectrograph and/or increasing the pixel width of the array detector.
Signal sensitivity refers to the proportion of the signal response light from the sample that reaches and is recorded by the detector. Again, there are technical trade-offs between sensitivity and other performance factors. The technology employed to increase resolution often reduces signal throughput. The signal-gathering optics can also be a limiting factor, especially fiber-optics that capture a very low solid angle of light from the sample. High signal sensitivity is only critical for sample substances that are weak Raman scatterers.
Many sample substances do not respond to the laser light with fluorescence emission, however, when fluorescence light is emitted from the sample it is often so much more intense than the Raman response that the Raman spectrum of the sample cannot be observed at all. The more important it is to be able to obtain Raman spectra from a wide variety of substances, the more critical it is to employ some technical strategy for reducing the magnitude of fluorescence or the probability of fluorescence.
Fluorescence is arguably the greatest challenge to the application dependability of Raman spectrometry and many solution have been used to solve this problem. Following is a brief description of each including their shortcomings when applied to PRMI.
Multiple laser systems: If serious fluorescence-interference exists for a specific target material with a specific Raman laser, one well-established solution is to configure the Raman spectrometer with more than one laser. Since fluorescence occurs over a specific range of wavelengths, one can simply switch to a laser wavelength that does not excite fluorescence (at least over the spectral range examined by Raman spectrometry). Since some fluorescent centers produce fluorescence over a broad range of wavelengths and since some minerals contain multiple fluorescent centers, it could easily require more than two laser wavelengths to ensure a high likelihood of being able to obtain a Raman spectrum for a wide variety of targets (minerals species). Some Raman systems are configured with 4 or 5 lasers. The limitations of this approach for PRMI are expense and complexity. Employing even two lasers requires much more training and experience to operate and typically results in a Raman system that costs 50% to 90% more than the same model configured with only one laser. Raman systems with 4 and 5 lasers require considerable space, cost, and maintenance.
Sequentially shifted excitation: This type of fluorescence-rejection solution involves either an adjustable-wavelength laser or a dual-wavelength laser. Two Raman spectra are collected using two laser source wavelengths that are very close together. The two spectra are processed with the assumption that Raman features occur in the same position in each spectrum and fluorescence features shift an amount equal to the difference between the source wavelengths. This approach is only slightly less expensive than a multiple-laser system since it avoids the duplication of lasers and other spectrometer components. The limitations of this approach for PRMI are spectral artifacts and breadth of application. While analysis time (the time to collect two spectra instead of one) is also a factor, some systems can collect the two spectra concurrently. Shifted excitation systems are known to produce some non-Raman spectral artifacts out of the spectrum-processing employed. In addition, the greatest limitation is probably the fact that shifted-excitation is ineffective for samples producing fluorescence orders of magnitude brighter than Raman intensities. In these cases the Raman peaks have similar intensity (or less) than electronic noise and the “shot noise” typical of photon counting and cannot be extracted from the dual spectra.
Gated Raman: Gated Raman takes advantage of the difference in time scale in which Raman scattering and fluorescence occur. Employing a high-speed pulsed laser and a fast gating detector, such a Raman system is carefully timed so that the detector is open when the laser pulse hits the sample and closes within a few hundred ps before most or sometimes any fluorescence light can be generated. The laser and detector are controlled to wait until any fluorescence is likely to have decayed and then repeat this excitation/detection pattern as many times as possible-thousands of times per second. The limitations of this approach for PRMI are expense and sensitivity. Until recently all gated-Raman systems were expensive and complex custom-made optical-bench systems. Even the commercial gated Raman system that is now available costs three times as much as non-gated system of otherwise-equivalent sophistication. Additionally, an important implication of the mode of operation of a gated system is that, even if it is operated at a pulse frequency of 100 kHz, it spends only 5% of every second with the detector open and collecting data. Such a system may have to collect data for an impractical period of time for the many minerals that are weak Raman scatterers.
IR Lasers: Many Raman instrument makers offer systems with an IR laser source (e.g., 1064 nm) because there is little likelihood that such a long wavelength source will excite fluorescence. The primary limitation of this approach for PRMI is sensitivity. As stated above, the probability of Raman scattering increases as the wavelength of the excitation source decreases. In fact, Raman intensity increases with the wavelength to the fourth power. This means as the source wavelength gets longer Raman intensities from the same subject drop quickly. Since many minerals are weak Raman scatterers, the IR laser Raman system has limited application to minerals as a whole.
UV lasers: Raman systems employing UV lasers are relatively rare due to their spectral performance challenges and expense. A UV Raman system has to examine a narrow spectral region, making it technologically difficult to achieve adequate spectral resolution and the minimum Raman shift of such systems is typically 350 to 450 nm. The limitations of UV Raman for PRMI are expense and performance. The components required to meet the performance challenges in the UV are expensive and many, many minerals have important Raman peaks at Raman shifts below 400 nm.
Anti-Stokes Raman: In detail, the Raman Effect encompasses wavelength shifts to longer wavelengths (Stokes Raman) and wavelength shifts to shorter wavelengths (Anti-Stokes Raman). Since, in theory, fluorescence produces light at longer wavelengths than the excitation source, it has been proposed that fluorescence-interference can be avoided by configuring a Raman spectrometer to detect Anti-Stokes Raman scattering. The limitations of Anti-Stokes Raman include shorter wavelength fluorescence and Raman intensity limitations. While it is true that narrow-range fluorescence from inner-shell electrons does not occur at wavelengths longer then the excitation source, it has been found that broad-range fluorescence resulting from excited outer-shell electrons extends significantly into wavelengths shorter than the excitation source due to the complex interactions between these electrons through their relaxation pathways. Additionally, Anti-Stokes Raman depends on some of the quantum vibrational states within the target sample being already at an energy level above their ground state when excited by photons from the Raman laser. As a result, Anti-Stokes Raman intensities tend to be weaker than Stokes Raman and Anti-Stokes Raman intensities steadily decrease as the Raman-shift of peaks (from the same sample) increases.
Furthermore, it has become very popular to produce hand-held analyzers for use in field applications. However, this becomes physically and technologically impractical for Raman systems using a shorter wavelength laser source partially due to the physical size of the spectrograph component required to achieve useful spectral resolution. One challenge for a portable/field Raman system is to provide for laser light delivery, signal light collection, and sample visibility for aiming through a single optical port. Many hand-held units and some portables do not even try to provide for optical analysis aiming and depend upon the analysis site being large enough to simply point the optical front end of the analyzer at it. Additionally, a Raman analyzer for PRMI must be efficient at signal light collection to work successfully with weak Raman scatterers and field geologic applications often have a need to analyze mm size mineral grains.
Accordingly, there is a need for a more dependable, efficient, and cost-effective Raman Spectroscopy system for PRMI that avoids the fluorescence-interference problem. Additionally, it is desired that a portable PRMI system be available for in-field or in-house applications.